Actually, they're not working without a "ground". You still have to have a
current "sink" for the current on the shield.
End-fed half waves are common enough. Feeding at the end means a network to
feed the antenna at a voltage feed. Typically a parallel tuned circuit with one
end to "ground" and the other to the 1/2 wave. It is fed from coax a few turns
up from the bottom with the shield to the "ground" end of the coil.
The half wave end-feds I have seen were 2 meter jobs. At the low current levels
experienced with hw endfeeds, the capacitance from the magmount to the car roof
is more than adequate to provide a ground. Lacking that, the feedline shield
carries the current.
A half-wave on CB is over 16 feet. Don't recall seeing any true end-fed
halfwave whips on CB. Would clang on interstate overpasses, for one thing. It
would have to have been center loaded to get the feed impedance up. The trick
is to fix it so you don't have to have a *high* current connection to ground. A
reasonable 1/2 wave fake would have a loading coil in the center of the whip to
turn it into an electrical 1/2 wave.
Guy, K2AV
>
> From: Robert Thain <g0hgw@yahoo.com>
To: <towertalk@contesting.com>
> Date: 2003/03/18 Tue AM 07:49:02 EST
> To: towertalk@contesting.com
> Subject: [TowerTalk] End fed vertical HELP PLEASE.
>
> Hello (again),
>
> I've received just one or 2 replies on my original
> mail, (thanks for those).
>
> How do those end fed 1/2 wave "CB" verticals work,
> with no ground ?
>
> The closest I have found is a J-pole, which is close
> but not what I'm looking for.
>
> To end feed a 1/2 wave it has to be a high voltage
> feed and not the traditional current feed.
>
> I remember from a long time back that these antennas
> had a little air spaced coil at the feed point and no
> ground radials etc.
>
> Help please,
>
> Robert
> G0HGW
>
>
>
>
>
> __________________________________________________
> Do You Yahoo!?
> Everything you'll ever need on one web page
> from News and Sport to Email and Music Charts
> http://uk.my.yahoo.com
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
> questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
|